Resistance elements of the type contemplated by us may be used as fusible leads forming part of an electrically programmable read-only memory (EPROM), designed to be selectively destroyed by a burn-through procedure in which they are traversed by an elevated electric current. Such a memory, as is well known in the art, may comprise a two-dimensional matrix of cells each consisting, in the simplest case, of a single active semiconductor component connected to a respective reading wire via fusible lead of the type referred to. In integrated circuitry, such a lead usually consists of a resistance element of metal or doped semiconductor material, e.g. polycrystalline silicon, designed to carry small operating currents well below the fusion level used for programming.
Conventionally, such resistance elements are manufactured by thin-film technique as conductor strips on an insulating substrate. Their resistance is determined by taking into account the various electrical and structural parameters such as the maximum voltage which can be tolerated by the memory cells during programming, the available surface area, and the resistivity of the conductor material.
In practice, these fusible leads are not exactly identical in view of unavoidable manufacturing tolerances. Thus, the destructive current employed in programming is to be chosen on the basis of the minimum resistance of all the fusible leads in the array. This minimum resistance, therefore, must not be so low as to require a burn-through current of a magnitude that may impair the active memory components.
One of the factors significantly influencing the resistance of these leads is their width. With thin-film conductor strips, that thickness cannot be made less than two to three microns with today's manufacturing procedures. By way of example it may be mentioned that, in order to rupture such a conventional fusible lead with a resistance of 1000 ohms, a current pulse of 30 mA and a duration of 0.3 .mu.sec is needed.